An intervertebral endoprosthesis is known (EP-B-176,728) which consists of two connection plates which between them enclose a core. The connection plates are intended to bear on the upper plates of the vertebral bodies on either side. To ensure that the prosthesis does not lose its predetermined position between the vertebral bodies as a result of shearing forces which principally occur in the anteroposterior direction parallel to the main extension of the connection plates, the connection plates must be connected to the vertebral bodies in a suitable manner. For this purpose, the edges of the known prosthesis are provided with teeth which project transversely from each connection plate in order to penetrate into the upper plates of the vertebral bodies under the natural force of pressure acting between said vertebral bodies. In this state, they provide resistance to displacement of the connection plates in relation to the upper plates of the vertebral bodies in any given surface direction of the connection plates and vertebral bodies. To ensure that this resistance is relatively great, the teeth, in the practical design of, said endoprosthesis which has become familiar through use, are arranged transverse to the direction of the surface extension of the connection plates and upper plates. They are in fact delimited by a pair of essentially triangular main surfaces which are attached to the connection plate along their essentially parallel base sides and whose free tip lying opposite the base side points away from the connection plate. For the teeth to have sufficient stability, they are designed with a certain thickness which corresponds to the distance between the triangular surfaces lying parallel to one another. The edges of the triangular surfaces, which extend between the base side and the tip, are connected to each other by rectangular, plane surfaces which converge toward the tooth tip and there form a ridge.
When using the prosthesis, it has been found that the teeth in many cases do not penetrate sufficiently deeply into the upper plates of the vertebral bodies. There is then a risk that they will not be able to correctly perform their function, namely that of holding the prosthesis in the intended position. Experience has shown that in such cases recourse must be made to the use of prosthesis pegs which are inserted into holes which have been drilled beforehand in the vertebral bodies (DE-C-3,023,353, FR-A-2,659,226). However, such working of the vertebral bodies is difficult. It prolongs the operation and weakens the vertebral bodies. The use of pointed, pyramid-shaped teeth is ruled out because the teeth become thicker towards the base, for the purpose of tooth stability, and are accordingly delimited by oblique surfaces, which, in the event of shearing forces, leads to the teeth lifting from the bone.
The object of the invention is to develop the intervertebral endoprosthesis mentioned in the introduction in such a way that the teeth penetrate sufficiently securely into the upper plate of the respective vertebral body and are also securely anchored therein against the effect of shearing forces.
Accordingly, an intervertebral endoprosthesis is proposed which has a connection plate which is to be fixed so as to bear on a vertebral surface. This connection plate is to be applied to the vertebral surface in a direction of application transverse to its main extension. In order to protect against a shearing force running parallel to its main extension, the connection plate has teeth projecting approximately in the direction of application. These teeth are delimited inter alia by a main surface arranged in the direction of application and transverse to the direction of shearing. In contrast to the conventional, pyramid-shaped tooth configuration, in which the surfaces delimiting the tooth are inclined at the same angle to the direction of application, the teeth according to the invention are designed skew in relation to the direction of application, by means of the fact that the main surface runs parallel to the direction of application and the widening of the tooth from the tip toward its base, as is necessary for its stability, is obtained solely by skewing of the other surfaces.
A non-generic hip-joint endoprosthesis is admittedly known (FR 2,641,461 A1) whose hemispherical part to be anchored in the hip bone provides, on the outer edge, a ring of wedges which converge to a point in the direction of insertion. The outer surfaces of the wedge bodies lie on a cylindrical surface whose cylinder axis runs in the direction of application. However, it remains unclear what technical effect if any is achieved with the cylindrical configuration of the outer wedge surfaces.
The invention is further characterized by the fact that two teeth or groups of teeth in each case lie opposite one another in the direction of shearing and point in opposite directions. This means that the main surfaces of two associated tooth pairs are directed either facing each other or facing away from each other. The reasoning behind this measure lies in the fact that the forces which run transverse to the direction of application, and which are generated when the oblique surfaces penetrate into the vertebra, are oriented in opposite directions and therefore cancel each other out. The teeth can therefore penetrate into the vertebra in the intended direction of application, i.e. in the direction of the main surfaces.
The configuration of the teeth according to the invention has two main advantages. One is that when a shearing force occurs parallel to the main extension of the connection plate, there is always at least one tooth main surface which runs perpendicular to the direction of this shearing force and therefore affords the best conditions for securing the position of the implant against this shearing force. The risk which is present with pyramid-shaped teeth, namely that the implant will be lifted from its bearing position on the vertebra by moving in the shearing direction along an oblique tooth surface, is eliminated.
The second advantage lies in the fact that the vertebral surfaces with which the main surfaces of the teeth cooperate during penetration, and on which they are supported against shearing forces, are spared. Tooth surfaces which run obliquely in relation to the direction of application have the effect that as they penetrate into the vertebral cortex, they cause the latter to gradually break open in order to accommodate the increasing cross section of the penetrating tooth. This breaking impairs the stability of the part of the vertebra concerned. By contrast, at the main surfaces of the tooth which extend in the direction of application, there is no gradual breaking. The parts of the vertebra concerned are therefore spared, remain more stable and better support the tooth. The integrity of the vertebral cortex of the upper plates, and its ability to take up forces, is better preserved in the area of the penetrating teeth, and for this reason it is generally possible to dispense with additional securing means such as screws.
This is especially true if the edges of the main surface are sharp, so that as they penetrate into the bone they cut the latter rather than break it open. If the main surface is plane, the sharpness of the edges of the main surface can be obtained by those surfaces of the tooth which are adjacent to the main surface being arranged at an acute angle (in a cross section parallel to the main extension of the connection plate). The base of the tooth is in this way given, for example, a triangular or trapezoidal configuration. However, it is not absolutely necessary for the main surface to be plane; instead, it can also be made up of two or more plane surfaces adjoining each other at a preferably obtuse angle, or it can be designed with a more or less uniform curvature.
The direction of application is the direction at which one would normally seek to apply the implant to the vertebra. If the connection plate is flat, it is normally perpendicular to the plane of the vertebral plate. If the connection plate is curved, the direction of application is generally by nature perpendicular to the average direction of extension of the connection plate. It can also be seen from the product as the direction at which the main surfaces run parallel to each other.
The shearing forces against which the teeth are intended to hold the intervertebral endoprosthesis on the vertebra generally occur as components of irregularly directed forces, with force components in the anteroposterior direction being particularly high and posing a particular threat to the fixing of the implant on the vertebra. It is therefore particularly important to fix the connection plates in the anteroposterior direction relative to the upper plates of the vertebral bodies. The main surfaces of the teeth provided on the connection plates of the implant are therefore oriented in the A-P direction and in adjoining directions. It is recommended to provide two groups of teeth, of which one group provides support against the anteriorly directed forces and the other group provides support against the posteriorly directed forces. These groups are arranged pointing in opposite directions in such a way that the forces running transverse to the direction of application, and generated by their oblique surfaces as they penetrate into the bone, essentially cancel each other out.
In this connection, a symmetrical arrangement in relation to an axis of symmetry extending transverse to the direction of the shearing force, or in relation to a mirror-symmetrical centre point, may be particularly suitable; however, symmetry is not necessary.